Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Each electrode is in intimate contact with a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
A variety of metals, ceramics, carbons, and composites have been studied for use as electrodes for ultracapacitors. Carbon based electrodes are common and are widely used in commercially available devices. Carbon has a low atomic weight and carbon electrodes can be fabricated with very high surface areas. The present invention relates to an ultracapacitor cell that has a uniform porous carbon electrode and to a method of fabricating the electrode for use in a high performance double layer capacitor.
Numerous methods are known to make a carbon electrode. For example, an electrode can be fabricated by a forming process, by pressing electrode materials in a die or by slurry pasting with binders or screen printing carbon as a paste with a liquid phase binder/fluidizer. Both dry and wet electrode formations may include a binder such as polymers, starches, Teflon.RTM. particles or Teflon.RTM. dispersions in water.
Slurry pasting does not always produce a uniform and reproducible electrode. A nonuniform electrode results in a nonuniform voltage distribution, which adversely affects ultracapacitor operation. Further, the presence of electrochemically inert binders decreases ultracapacitor energy density. The present invention relates to a process for nonaqueous electrode fabrication using an organic diluent/electrolyte based knife pasting process. In the inventive process, liquid electrolyte functions as a binder to hold carbon particles together to eliminate the need for other binders. Further, the knife pasting process according to the invention results in improved reproducibility. The invention results in uniform electrodes to eliminate nonuniform voltage distribution. The present invention is advantageous in utilizing an organic diluent, preferably of a low boiling point, that can be easily evaporated to form uniform paste electrodes. Finally the invention provides low cost electrodes.